Blue-violet semiconductor lasers which are produced by using III-V group nitride semiconductor materials (AlxGayIn1−x−yN (0≦x≦1, 0≦y≦1)), such as gallium nitride (GaN), are key devices for realizing ultrahigh density recording in optical disk apparatuses, and now are reaching a practical level. Increasing the output of a blue-violet semiconductor laser would not only enable rapid writing to an optical disk, but would also be an indispensable technology for exploring new technological fields, such as applications to laser displays.
In recent years, GaN substrates are regarded promising as substrates which are necessary for producing nitride semiconductor devices. GaN substrates are superior to the conventionally-used sapphire substrates in terms of crystal lattice matching and heat-releasing ability. Another advantage is that GaN substrates are electrically conductive, whereas sapphire substrates are insulators. In other words, an electrode may also be formed on the rear face of a GaN substrate, thus adopting a structure in which a current will flow in a direction across the GaN substrate. Forming an electrode on the rear face of an electrically-conductive GaN substrate makes it possible to reduce the size (chip area) of each individual semiconductor device. A reduced chip area means an increased total number of chips that can be produced from a single wafer, whereby the production costs can be reduced.
A semiconductor laser having an n-side electrode which is formed on the rear face of a GaN substrate is disclosed in Patent Document 1, for example.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-16312
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-71657
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2004-6718